Recent advances in polymerization and catalysis have resulted in the ability to produce many new polymers having improved physical and chemical properties useful in a wide variety of superior products and applications. Regardless of these technological advances in the polyolefin industry, common problems, as well as new challenges still exist.
Advances in polymerization technology have provided more efficient, highly productive, and economically enhanced processes. Especially illustrative of these advances is the development of single site catalyst systems. Single site catalysts have been shown to be highly useful in the preparation of polyolefins, producing relatively homogeneous copolymers at good polymerization rates, allowing one to closely tailor the final properties of the polymer produced. In contrast to traditional Ziegler-Natta catalyst compositions, single site catalyst compositions comprise catalytic compounds in which each catalyst composition molecule contains one or only a few polymerization sites. Metallocenes are the most well known type of single site catalyst precursor, and are organometallic coordination complexes containing one or more pi-bonded moieties (e.g., cycloalkadienyl groups) in association with a metal atom from Groups IIIB to VIII or the Lanthanide series of the Periodic Table of Elements.
There has been tremendous focus in the industry on developing new and improved metallocene catalyst systems, designing the catalyst systems to produce new polymers, to improve operability or processability, and to improve catalyst productivity. Metallocene catalyst variables include the metal atom, the ligands or pi-bonded moieties associated with the metal atom, the type of catalyst support used, as well as any catalyst activator and the reduction ratio of the catalyst or catalyst precursors.
Unfortunately, improving upon the productivity, operability, processability, and economics of a polymerization process entails overcoming several obstacles, especially when introducing a new catalyst. A few of these issues are discussed in patents and published patent applications including: U.S. Pat. No. 6,339,134, discussing ways to improve melt properties of metallocene produce polymers; U.S. Pat. No. 6,987,152, discussing the effect of feed impurities upon the process; U.S. Pat. No. 6,914,027, discussing fines production, sheeting/fouling, and their related heat transfer problems; and U.S. Patent Application Publication No. 2005/0137364, discussing problems associated with heat removal from the polymerization reactor; each of which is hereby incorporated by reference. Each of these references, among others, describes common problems encountered and which are of concern when introducing a new catalyst to the process due to the unpredictability of results associated with a new catalyst or catalyst system.
For example, the processability of metallocene and metallocene-type catalyzed polyethylenes (mPE) is often different than polyethylenes produced in a high pressure polymerization process or with other catalysts, such as Ziegler-Natta type catalysts. For example, mPEs typically have a narrow molecular weight distribution, which tend to be more difficult to process. Generally, the broader the polymer molecular weight distribution, the easier the polymer is to process. These examples illustrate the challenges to be encountered when commercializing a new catalyst system.
Generally, these mPEs require more motor power and produce higher extruder pressures to match the extrusion rate of LDPEs. Typical mPEs also have lower melt strength which, for example, adversely affects bubble stability during blown film extrusion, and they are prone to melt fracture at commercial shear rates. On the other hand, mPEs exhibit many superior physical properties as compared to LDPEs.
Additionally, reactor conditions and the catalyst employed in the polymerization affect numerous physical and chemical properties of the polymer, including molecular weight, molecular weight distribution, compositional distribution, crystallinity and melting temperature, and extractable content (e.g. hexane extractables), among others. In addition to the several reactor and reactant process control variables which may be manipulated during production, polymer product properties may also vary based upon catalyst formulation and structure. The metal atom and the ligands (pi-bonded moieties) forming the metallocene complex can affect the properties of the polymer product formed. The support architecture, the number of functional groups on the support (such as —OH groups on silica), the activator loading, and the pre-impregnated catalyst loading can also affect the product formed.
End users often desire improvements or a balancing of several polymer properties. Among these are included melting point for a given density, tear properties, impact and tensile strength, heat seal and hot tack properties, and others. For example, there is a strong desire in the industry to improve heat seal and hot tack properties in PE films. It is particularly desirable to lower the heat seal temperature, broaden the hot tack window and increase the hot tack strength while maintaining low extractables to meet regulatory requirements for food packaging. These improvements are usually accomplished by lowering the density of the film resin. This, however, may negatively affect other film properties such as tear strength, dart impact strength, stiffness, and it lowers the melting temperature of the film. Additionally, to achieve good clarity, low haze, and better processability, metallocene resins are often blended with high pressure LDPE, adding to manufacturing costs.
Other background references include EP 1 300 240 A, WO 1996/00245, WO 2003/008468, WO 2003/093332, WO 2004/000919, U.S. Patent Application Publication Nos. 2005/0058847, 2005/054791, 2005/288443.
Accordingly, there exists a need for process improvements, including catalyst technology, that improve process operability, polymer properties, and/or polymer processability. There also exists a need for a metallocene catalyst that allows for increased process flexibility, where the metallocene can be used to produce multiple grades over wide density and melt flow ranges, minimizing catalyst transitions while meeting product specifications for specific end uses such as rotomolding and injection molding.